In a gas phase process for production of polyolefins such as polyethylene, a gaseous alkene monomer (e.g., ethylene, propylene, etc.), hydrogen, co-monomer and other raw materials are converted to solid polyolefin product. Generally, gas phase reactors include a fluidized bed reactor, a compressor, and a cooler. The reaction is maintained in a two-phase fluidized bed of granular polyethylene and gaseous reactants by a fluidizing gas, which is passed through a distributor plate near the bottom of the reactor vessel. The reactor vessel is normally constructed of carbon steel and rated for operation at pressures up to about 30 bars (or about 3.0 MPa). Catalyst is injected into the fluidized bed. Heat of reaction is transferred to the circulating gas stream. This gas stream is compressed and cooled in an external recycle line and then is reintroduced into the bottom of the reactor where it passes through a distributor plate. Make-up feed streams are added to maintain the desired reactant concentrations.
Operation of most reactor systems is critically dependent upon good mixing in the fluidized bed for uniform reactor conditions, heat removal, and effective catalyst performance. Good mixing is required to ensure that the catalyst is well distributed within the bed so that the reaction rate and resulting heat generation is relatively uniform, thereby minimizing the possibility of localized temperature excursions (or “hot spots”) within the bed.
The process must be controllable, and capable of a high production rate. In general, the higher the operating temperature, the greater the capability to achieve high production rate. However, as the operating temperature approaches the melting point of the polyolefin product, the particles of polyolefin become tacky. This can cause the fluidized bed (as a whole) to become cohesive, or sticky. If the temperature exceeds certain limiting temperatures (dependant on the melting point of the polymer involved) the degree of stickiness in the fluidized bed may become excessive, causing poor fluidization and mixing. In some cases, the sticky polymer and resulting lack of mixing can lead to localized temperature excursions of sufficient magnitude to cause the formation of particle agglomerates (or chunks) of fused polymer in the reactor. In other cases, the sticky polymer and poor mixing can promote the formation of polymer sheets on the interior walls of the reactor.
Poor mixing of the fluidized bed (and the consequent potential for chunk or sheet formation) may also be caused by distributor plate fouling. Distributor plate fouling is one of the leading causes of downtime with commercial fluidized bed polymerization reactor systems. Fouling is generally caused by deposition of polymer resin in the numerous small holes in the distributor plate, resulting in reduced fluid flow therethrough or complete blockage thereof. As mentioned above, good mixing of the fluidized bed is needed for uniform temperature control. As the holes in the distributor plate become partially or fully blocked, the ability of the cycle gas entering the fluidized bed to carry heat away from the reacting materials is reduced. Moreover, “hot spots” can develop in areas of low fluid velocity in the fluidized bed (particularly those areas immediately above the partially or fully blocked holes). The net result is the formation of fused chunks of polymer within the fluidized bed, and/or the formation of sheets along the vessel wall and along other parts of the reactor system. These chunks or sheets will eventually fall onto the reactor distributor plate, further interrupting fluidization, circulation of gas, and withdrawal of the product from the reactor. The result is a forced reactor shutdown to clean the system. The formation of chunks or sheets can therefore be a significant “discontinuity event”, impacting operations of commercial reactor systems. To minimize the possibility of chunk or sheet formation, it is important to prevent or minimize distributor plate fouling.
More recently, a particularly problematic form of plate fouling (termed hyperfouling) has been observed, which can occur during reactor startup. While the precise cause is not completely understood, high levels of entrainment static (measured in the cycle gas system) are observed upon initiation of catalyst feed to the reactor. This static is attributed to entrainment of catalyst particles from the fluidized bed and consequent triboelectric charging of the catalyst particles by frictional contact with the walls of the recycle system. The charged catalyst particles can be driven to the reactor walls by forces of static attraction, where they can accumulate (especially under the plate and/or top head of the reactor) and fuse to form foulant. This mechanism is supported by the observation of temperature spikes (above net reactor temperature) in the bottom bell of the reactor (below the distributor plate).
Conventional wisdom is that low cycle stream velocities (and correspondingly low superficial gas velocities in the fluidized bed) reduce distributor plate fouling by minimizing entrainment of solids in the cycle stream, thereby minimizing contact of such solids with the distributor plate.
Current methods for removing distributor plate fouling require shutting down the reactor and physically removing the foulant from the holes, such as with a drill. Not only are such shutdowns costly in terms of lost production, but may also pose a danger to the operator entering and working in the reactor system.
Accordingly, it would be desirable to reduce and/or remove distributor plate fouling without requiring system shutdown.